1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a semiconductor device in which low forward voltage VF and low reverse current IR characteristics of a Schottky barrier diode are improved.
2. Description of the Related Art
FIGS. 7A and 7B are sectional views of conventional Schottky barrier diodes D2 and D3.
In the Schottky barrier diode D2 of FIG. 7A, an n−-type semiconductor layer 32 is laminated on an n+-type semiconductor substrate 31, a guard ring 34 for securing breakdown voltage at the time of applying reverse voltage of the Schottky barrier diode D2 is provided at the periphery, and a Schottky metal layer 36 of Mo or the like for forming a Schottky junction with the surface of the semiconductor layer 32 is provided.
An anode electrode 37 is provided on the Schottky metal layer 36, and a cathode electrode 38 is provided on the back surface of the substrate 31. Current flows at the time of applying forward voltage, and current does not flow at the time of applying reverse voltage by Schottky barrier.
Forward voltage VF as a rising voltage of the Schottky barrier diode D2 and leak current IR at the time of applying the reverse voltage are determined by a work function difference (hereinafter referred to as φBn) obtained at the Schottky junction between the Schottky metal layer 36 and the surface of the semiconductor layer 32. In general, there is a relation of trade-off in which when φBn is high, the forward voltage VF becomes high, and leak current IR is lowered.
The Schottky barrier diode D3 having a structure shown in FIG. 7B is also known.
In the Schottky barrier diode D3, an n−-type semiconductor layer 22 is laminated on an n+-type semiconductor substrate 21. The resistivity of the n−-type semiconductor layer 22 is, for example, about 1 Ω·cm when the diode is a 40V series device.
A plurality of p+-type regions 23 are formed in the semiconductor layer 22 by diffusion of high concentration p-type impurities or the like. The interval of the adjacent p+-type regions 23 is such a distance that a depletion layer is pinched off.
In order to secure the breakdown voltage at the time of applying the reverse voltage of the Schottky barrier diode D3, a guard ring 24 is provided by diffusion of high concentration p-type impurities or the like to surround the outer periphery of all the p+-type semiconductor regions 23. All the p+-type semiconductor regions 23 disposed inside of the guard ring 24 and the surface of the semiconductor layer 22 are in contact with a Schottky metal layer 26.
The Schottky metal layer 26 is made of, for example, Mo, and forms a Schottky junction with the surface of the semiconductor layer 22. For example, an Al layer is provided as an anode electrode 27 on the Schottky metal layer 26, and a cathode electrode 28 is provided on the back surface of the n+-type semiconductor substrate 21.
In this case, since the Schottky metal layer 26 can be regarded as a false p+-type region, the Schottky metal layer 26 and the p+-type regions 23 can be regarded as a continuous p-type region.
In the Schottky barrier diode D3, when forward voltage is applied, current flows. On the other hand, when reverse voltage is applied, the depletion layer is extended by the pn junction between the combination of the p+-type regions 23 and the Schottky metal layer 26, and the n−-type semiconductor layer 22. At this time, a leak current corresponding to the kind of the Schottky metal layer 26 is generated at the interface between the semiconductor layer 22 and the Schottky metal layer 26.
However, since the p+-type regions 3 are disposed at intervals of such a distance that the depletion layer is extended and is pinched off, the leak current generated at the interface is suppressed by the depletion layer, and leak to the cathode electrode 28 side can be prevented.
That is, while a characteristic is held in which a specified forward voltage VF can be obtained, it is possible to suppress the increase of the leak current (IR) due to the increase of the reverse voltage (VR) (see, for example, Japanese Laid Open Patent Publication No. 2004–127968).
When the diodes having the same chip size (Schottky junction area) are compared with each other, the actual Schottky junction area (area of the n−-type semiconductor layer) of the Schottky barrier diode D3 is smaller than that of the Schottky barrier diode D2 (FIG. 7A).
In general, in the case where the same resistivity of the n−-type semiconductor layer and the same Schottky metal layer are provided, when the Schottky junction area is small, the forward voltage VF becomes high.
Besides, in the Schottky barrier diode D3, since the n−-type semiconductor layer between the adjacent p+-type region 23 becomes a current path, the resistance of a region above the n−-type semiconductor layer becomes higher than that of the Schottky barrier diode D2 (FIG. 8A).
That is, in the case where the chip size, the resistivity ρ1 of the n−-type semiconductor layer and the Schottky metal layer are the same, when the structure of the Schottky barrier diode D3 is adopted, the forward voltage VF becomes high.
It is conceivable that the resistance value of the narrow current path is reduced by lowering the resistivity ρ1 of the n−-type semiconductor layer 22 to achieve lowering the forward voltage VF. However, in this method, the resistivity of the n−-type semiconductor layer 22 below the p+-type region 23, which determines the breakdown voltage, is also lowered. Accordingly, there has been a problem that the extension of the depletion layer becomes insufficient, and a specified breakdown voltage can not be secured (FIG. 8B).